In less than one year after the discovery of X-radiation by Roentgen in 1914 silver halide emulsions were being used in radiographic medical diagnostic film. It was recognized almost from the very outset that the high energy ionizing X-radiation was potentially harmful, and ways were sought to avoid high levels of patient exposure.
One approach, still in wide-spread use was to coat the silver halide emulsions on the opposite faces of the film support. It was recognized that a silver halide emulsion layer absorbs only about 1 percent of the X-radiation it receives. By coating a second emulsion layer on the back side of the support X-radiation absorption can be doubled. Dual-coated radiographic films are sold by Eastman Kodak Company under the trademark "Duplitized".
A second approach for X-ray dosage reduction that is compatible with the first approach is to rely on a phosphor containing X-ray intensifying screen to absorb X-radiation and to emit light that exposes the silver halide emulsion of the radiographic element. X-ray intensifying screens are approximately 20 times more efficient in capturing X-radiation than silver halide emulsions. In 1918 the Eastman Kodak Company introduced the first medical radiographic product that was dual coated, and the Patterson Screen Company that same year introduced a matched intensifying screen pair for that product.
As would be expected, indirect radiographic films, those in which an intensifying screen is relied upon to capture X-radiation and to emit light that exposes the film, are fundamentally different in their construction from direct radiographic films, in which imaging depends on the silver halide grains to absorb X-radiation. The primary function of the silver halide grains in indirect radiographic films is to capture light and to produce a viewable silver image. Hence the silver halide coating coverages of dual-coated indirect radiographic films are typically in the range from 1.5 to 3.0 g/m.sup.2 of silver per side. About the same overall silver coverage levels are employed in comparable single-sided films (films with silver halide emulsion coatings on only one side of the support).
To at least partially compensate for the much lower X-ray absorption capabilities of silver halide emulsions as compared to intensifying screens direct radiographic films are coated at much higher silver coverages than indirect radiographic films. A typical coating coverage for a dual-coated direct radiographic film is approximately 5 g/m.sup.2 of silver per side, with about the same overall silver coverage levels for single-sided direct radiographic films.
In addition to the two broad categories of silver halide radiographic films noted above there is a third category of radiographic film, most commonly employed for dental intra-oral diagnostic imaging and hereafter referred to as dental film. Intra-oral dental imaging has presented practical barriers to the use of intensifying screens. Thus, dental films rely on silver halide grains for absorption of X-radiation. However, the levels of silver coverage typical of general purpose direct radiographic films noted above are inadequate for dental diagnostics. Because of the small size of dental defects sought to be detected, much lower levels of image noise (e.g., granularity) can be tolerated than for general medical diagnostic imaging applications. Thus, for dental films it is not the level of silver that will produce an acceptable maximum image density that controls silver coverages, as in indirect radiographic films, nor is it the level of silver that is capable of directly absorbing X-radiation in an amount sufficient for image generation, as in general purpose direct radiographic films. Dual-coated dental films require still higher silver coverages of greater than 7.5 g/m.sup.2 per side to produce silver images of acceptably low noise levels to satisfy the rigorous diagnostic demands of dentistry. The high silver coverages preclude constructing single-sided dental films.
Before 1950 the most commonly employed silver halide emulsions were those prepared by single-jet precipitations. In single-jet precipitations all of the halide salt solution is present in the reaction vessel before silver salt solution is introduced. Thus, precipitation begins with a large stoichiometric excess of halide ions that is continuously reduced as precipitation progresses. An unsought by-product of this precipitation approach is that some tabular grains are produced during the precipitation. No advantage was assigned to the presence of tabular grains, and, in fact, tabular grains all but disappeared from commercial silver halide emulsions when double-jet precipitation, the concurrent addition of silver and halide salt solutions, replaced single-jet precipitation as the emulsion manufacturing procedure of choice.
From 1937 until the 1950's the Eastman Kodak Company sold a dual-coated (Duplitized.sup.TM) direct radiographic film product under the name No-Screen X-ray Code 5133. Silver coverage was about 5 g/m.sup.2 per side. The product represents the highest proportion of tabular grains found in a single-jet emulsion. Tabular grains accounted for greater than 50% of the total grain projected area while nontabular grains accounted for greater than 25% of the total grain projected area. Based on remakes of the emulsion it was concluded that the tabular grains had a mean diameter of 2.5 .mu.m, an average tabular grain thickness of 0.36 .mu.m, and an average aspect ratio of from 5 to 7. The product that superseded Code 5133 to serve the same application was essentially free of tabular grains.
Kofron et al U.S. Pat. No. 4,439,520, filed Nov. 12, 1981, discovered significant photographic advantages for chemically and spectrally sensitized high (&gt;8) aspect ratio tabular grain emulsions. Speed-granularity improvements in both silver and dye-imaging applications were demonstrated. The importance of this discovery was immediately appreciated. Jones et al U.S. Pat. No. 4,478,929 demonstrated that by employing chemically and spectrally sensitized high aspect ratio tabular grain emulsions in dye image transfer systems silver coverages could be reduced from 1.3 g/m.sup.2 to 0.4 g/m.sup.2 with minimal loss of speed.
Concurrently Abbott et al U.S. Pat. Nos. 4,425,425 and 4,425,426 recognized that the use of chemically and spectrally sensitized high (&gt;8) and intermediate (5-8) aspect ratio tabular grains in dual-coated radiographic elements could be used to reduce crossover, a major source of image unsharpness in dual-coated indirect radiographic films.
Dickerson U.S. Pat. No. 4,414,304 recognized that the use of thin (&lt;0.2 .mu.m) tabular grain emulsions in single-sided or dual-coated indirect radiographic films could be used to reduce silver coverages. The silver coverage of indirect radiographic films is that required to achieve the desired maximum density. It was discovered that thin tabular grain emulsions exhibit increased covering power, defined as 100 times maximum density divided by silver coverage in g/dm.sup.2, in fully forehardened emulsions. The art had previously completed hardening during processing after exposure to minimize silver coverages. The practical effect of the discovery is that the practice of delayed hardening has greatly declined.
The discoveries of advantages for tabular grain emulsions has had no impact on dental films. The discoveries of Kofron et al and Abbott et al have no applicability to dental films, since spectral sensitizing dyes are relied upon to capture light while dental films are imagewise exposed only to X-radiation. The discovery of Jones et al relating to dye image transfer systems has no applicability to dental films, since the latter form only silver images. Insofar as the absorption of X-radiation by silver halide grains is concerned, it is immaterial what shape the silver halide grain takes. Absorption is entirely a function of the mass of silver coated, rather than the shape of the individual grains. Further, the granularity of direct X-ray images is a function of the number of grains coated per unit area rather than their shape.
Roberts et al U.S. Pat. No. 4,865,944 combines phosphors and tabular grain emulsions in an integrated intensifying screen and indirect X-ray exposure film intended to serve dental use. Unfortunately, in this construction the phosphors can be used only once. This has rendered this approach to dental imaging cost prohibitive.
Although dental films have continued to employ the emulsions in use prior to the discoveries relating to tabular grain emulsions, dental imaging has continued to experience problems that are peculiar to this application. Whereas silver halide radiographic films are generally processed in highly automated rapid access processors (e.g., Eastman Kodak Company's RP X-Omar.TM. processor), the small usage of dental film in terms of square meters has precluded the practical adaptation of general rapid access processing to use in dental offices. One of the practical concerns is that processing solutions often become seasoned over extended time and repetition of use, producing different image characteristics with the same film, depending on the stage of seasoning.